Method of removing a metal contaminant

ABSTRACT

THE PRESEENT INVENTION RELATES TO A METHOD OF CONDITIONING A METAL SUBSTRATE. MORE PARTICULARLY, THE PRESENT INVENTION RELATES TO A METHOD OF REMOVING A METAL CONTAMINANT PLATED ON A DIFFERENT METAL SUBSTRATE SUCH AS, FOR EXAMPLE, AN ELECTRODE USED IN A METAL ELECTRODEPOSITION PROCESS.

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METHOD OF REMOVING A METAL CONTAMINANT Filed Sept. 11, 1972 FIG.I

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mmaon or nwovme A METAL couummm med Sept. 11, 1972 5 Sheets-Sheet 5 E2: 2322. hzummao o 66 God .II'IIII-ll OOQ aos""a United States Patent U. S. Cl. 204- 140 7 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method of conditioning a metal substrate. More particularly, the present invention relates to a method of removing a metal contaminant plated on a different metal substrate such as, for example, an electrodeu's'ed in a metal electrodeposition process.

. BACKGROUND OF THE INVENTION Numerous processes are known for producin metals electrolytically. In such processes, at least two electrodes are placed in an electrolyte containing salts of the desired metal. An electric current is caused to flow from one electrode to the other electrode whereby the desired metal is plated upon the surface of one of the electrodes, generally the cathode. The metal plated electrode subsequently is removed from the electrolyte and the plated metal recovered therefrom.

An example of such a process is the electrodeposition of cobalt. Cobalt is electrodeposited on a stainless steel cathode and subsequently recovered by subjecting the deposit to an impact, causing the cobalt to break and fall off the cathode. Such a method of removal leaves minor amounts of cobalt on the cathode which, if not removed. will interfere with the uniformity of a subsequent deposit.

In the electrodeposition of copper the electrodes are placed in an electrolyte comprising a copper sulfate solution. After a desired deposit of copper has accumulated on one of the electrodes, that electrode is removed from the bath and the copper peeled or chipped from the electrode. Frequently, small amounts of copper, in the form of fragments or a film, will remain on the electrode as a contaminant. Such remaining amounts of copper will interfere with the uniformity of subsequent electrodeposits and increase the difficulty of removing a subsequent deposit from the electrode.

Manganese, like cobalt, also is deposited on a cathode and subsequently recovered by subjecting that deposit to a sharp impact. Further, as in the case of cobalt, such recovery leaves minor amounts of manganese on the cathode which interfere with the uniformity of subsequent electrodeposits.

Heretofore, the minor amounts of manganese have been removed from the cathode by placing the cathode in an acidic solution such as, for example, a sulfuric acid solu- H tion. Generally, the time required to remove the manganese by such a method is from about 3 to 6 hours. The cathode also is subjected to the action of the acid and results generally in corrosion and pitting ofthe cathode.

Attempts have been made to reduce the amount of damage done to the cathode by, controlling the temperature of the electrolyte, controlling the concentration of the acid, sparging the electrolyte with air, adding oxidizing chemicals to the electrolyte and the like. However, such attempts are subject to operator error and have not been SUMMARY OF THE INVENTION A method now has been discovered of rapidly removing a metal contaminant plated on a different metal substrate such as, for example, an electrode. Further, such method is accomplished under reproducible, controlled conditions. The method comprises providing an adjustable voltage electrodical power source, a potential monitoring means and an acidic solution having a reference electrode and an applied voltage electrode therein. The contaminant plated metal substrate is placed in the acidic solution and a potential between the reference electrode and the contaminant plated metal substrate is continuously monitored. A voltage is applied to the metal substrate and applied voltage electrode from the electrical power source, the voltage being sufficient to maintain a predetermined passive po tential between the metal substrate and the reference electrode, whereby the contaminant is rapidly removed from the metal substrate with substantially no detrimental effect on the surface of the metal substrate.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 5 are polarization curves.

DESCRIPTION OF A PREFERRED EMBODIMENT The present invention provides a method of removing a metal contaminant plated on a different metal substrate and is particularly applicable to an electrode used in a metal electrodeposition process. It has been found that the present invention removes a contaminant such as, for example, metal plated on an electrode. Further, it has been found that when the contaminant is removed in accordance with the instant invention the surface of the metal substrate suffers substantially no detrimental effect even though it is placed in a corrosive environment.

The term metal substrate as used herein contemplates any metal having a passive potential as defined hereinbelow. A particularly preferred group of metal substrates contemplated are the electrodes used in metal electroplating type processes such as, for example, cathodes used for the electrodeposition of cobalt, copper, manganese, zinc, chromium, nickel and the like. The present invention is applicable to any metal contaminant plated metal substrate which is less electro-active (more noble) than the metal contaminant.

It is known that a freely corrodiug metal in a corrosive solution develops an electrochemical potential. This potential results in a current flow which is indicative of the corrosion rate of the metal. A direct measurement of that current usually is not feasible. However, it is possibe to measure the current indirectly.

An indirect measurement of the current is obtained by placing a reference electrode in the solution with the corroding metal, measuring the potential between them, applying a voltage from a power source to the corrodiug metal,and measuring the current required to maintain a given potential between the corrodiug metal and the reference electrode. From the data so obtained itis possible to calculate the current density in the corroding. metal and to plot that measured potential versus current density. Such a plot generally is referred to as a polarization curve. A characteristic polarization curve is shown in FIG. 1.

As shown in FIG. 1, a typical polarization curve, there exists a range of anodic potentials (Points B to C) at which the material is deemed passive (it is undergoing a very low corrosion rate as shown by the low current flow). The term passive potential as used generally and herein refers to a potential within that range of potentials (Points B to C). For a more detailed description of this phenomenon see Polarization Data Yield Corrosion.

Rates, Chemical Engineering, July 26, 1971 and the references cited therein. Further, from the polarization curve it is seen that there exists a minimum current density that must be applied to the corroding metal (Point A) before the passive potential range is reached. That minimum current density generally is referred to as the instantaneous passivation current density.

Materials which are amenable. to anodic protection have an anodic polarization curve as shown in FIG. 1 and are referred to generally as active-passive materials. Examples of active-passive materials contemplated herein include nickel-chrome steels such as, for example, the 200, 300 and 400 series of stainless steel, as well as titanium, zirconium, and titanium or zirconium alloys.

Alternatively, many materials that do not exhibit an anodic passive range in a specific environment are amenable to cathodic protection. Broadly cathodic protection comprises applying suflieient electrical power to the metal to be protected to maintain a potential within the linear portion of the cathodic polarization curve (FIG. 1, Points D to B). As a general rule any material which exhibits a cathodic polarization curve having a linear portion with a slope greater than 0.060 is amenable to cathodic protection; the slope being defined as a ratio of the potential expressed in millivolts to the current expressed in milliamps. The term passive potential as used herein also includes that range of potentials (Points D to E) over which the cathodic polarization curve is linear and has a slope greater than 0.060. For a more detailed discussion of cathodic protection reference may be made to any of the numerous texts on the subject such as, for example, Corrosion Engineering, by Mars Fontana and N. D. Greene (1967), McGraw-Hill or Corrosion and Corrosion Control, by H. H. Uhlig (2nd Edition, 1971), John Wiley and Sons.

It is surprising to find that a metal plated substrate can be protected with the application of a controlled potential. More particularly, it is known that an interface between two dissimilar materials, having different corrosion potentials, results in severe localized corrosion at that interface. In accordance with the present invention a predetermined potential is maintained on the metal to be protected and a sufiicient current applied to maintain that potential.

For example, in the case of manganese plated on stainless steel, the potential at which the steel is passive results in a highly active or corrosive condition for the manganese. This condition necessarily results in a high current flow through the manganese-stainless steel interfaces.

Thus, it would be expected that such high current flow would, in turn, result in localized corrosion of the steel at those stainless steel-manganese interfaces. Surprisingly, for reasons not fully understood, such localized corrosion does not occur. Indeed, when a metal substrate is treated in accordance with the instant invention, substantially no difference is detected between an area that previously had no plating and one at which a plating interface existed.

The contaminant plated metal substrate is placed in an acidic solution. Obviously, as those versed in the art will appreciate, the acidic solution must be one in which the metal substrate has a passive potential as hereinbefore defined. The particular acidic solution used is not critical except that it is essential the acidic solution be capable of digesting the metal contaminant. Examples of applicable digesting acids include sulfuric, nitric,'phosphoric acids or organic acids such as acetic acid. A particularly preferred acidic solution, for economic reasons, is the ano lyte from which the metal contaminant was plated on the metal substrate.

The acidic solution should not contain an excessive concentration of halogen ions when an anodic passive potential is to be utilized as those versed in the art will appreciate. It is known that an excessive concentration of Cl ions will cause pitting when the metal substrate is a stainless steel or zirconium, for example. The exact concentration that can be tolerated is a function of the composi- H011 of the acidic solution, temperature, composition of the metal substrate and the like.

It has been discovered, however, that the passivation of titanium and its alloys is relatively unaffected by the presence of halogen ions. Thus, when the metal substrate is titanium or a titanium alloy the concentration of halogen ions such as CI, for example, is not critical.

In addition to the metal substrate, a reference electrode also is placed in the acidic solution. The reference electrode preferably is a standard reference such as'a saturated calomel electrode (SCE), silver/ silver chloride, mercury/ mercury sulfate and the like. Alternatively, the reference electrode may be formed from stainless steel, titanium or other metals that are not rapidly attacked by the acidic solution The essential characteristic of the reference electrode is that it not be readily attacked by the acidic solution and be capable of reaching an equilibrium potential under steady-state conditions in the acidic solution.

Further, there also is an applied voltage electrode in contact with the acidic solution. Advantageously, when the acidic solution is in a metal container that metal container is used as the applied voltage electrode. However, when the container is a non-conductive material such as fiberglass reinforced plastic or when the container is lined with a non-conductive material it is necessary to provide an applied voltage electrode in the acidic solution. The applied voltage electrode may be any material that is not rapidly corroded by the acidic solution such as, for exam ple, the alternate materials specified for the reference electrode.

A potential measurin means is provided to-permit con-. tinuous monitoring of the potential between the reference electrode and the metal substrate. Examples of suitable potential measuring means are high impedence measuring devices such as, for example, vacuum tube volt meters, potentiometers, null detectors and the like.

The adjustable voltage power source may be any of those known to those skilled in the art wherein the output voltage is adjustable and the power source will automatically vary the current supply to maintain that adjusted voltage. It is essential, however, that the power source be capable of supplying sufficient current to provide the instantaneous'passivation current density as hereinbefore defined, to the metal substrate if it is to be anodically protected. The purpose of the adjustable voltage power source is, of course, to provide sufiicient electrical power to maintain the metal substrate at a predetermined passive potential, as measured by the potential measuring means.

A particularly preferred system for use in practicing the method of the instant invention is one providing the functions of a potentiostat. A potentiostat combines the potential measuring means and the adjustable voltage power source into one unit. Further, it provides an additional feature in that a desired potential may be preset. The potentiostat then will monitor the potential between the reference electrode and the metal substrate while automatically adjusting the applied voltage to the applied voltage electrode and metal substrate to obtain and maintain that preset potential. Units or systems of this type are known to those skilled in the art. An example of such a system is disclosed in US. Pat. No. 3,127,337.

. The following examples are set forth for the purpose of illustrating the instant invention.

Example I A clean 316 series stainless steel plate (used asa cathode in a manganese electrodeposition process) is obtained and placed in an acidic solution having the following composition:

The temperature is maintained within the range of from 20 to 30 C. A polarization curve is determined as described herein before. From that curve, FIG. 2, it is seen that there is an anodic passive potential range of from about 300 to 800 millivolts with respect to a saturated calomel electrode (SCE) and the instantaneous passivation current density is about 50.0 milliamps.

That same stainless steel plate then is plated with an electrodeposit of manganese and subsequently placed in the above described acidic solution. The acidic solution contains, in addition to the stainless steel plate, a saturated calomel reference electrode and a stainless steel applied votlage electrode.

A potentiostat is connected to the manganese plated stainless steel plate, applied voltage electrode and refer-- ence electrode. The potentiostat is set to provide and maintain a passive potential of about 400 millivolts between the manganese plated plate and the reference electrode. The initial current density is about 62 ma. per square inch. After about 7 minutes the current density rapidly decreases to about 0.03 ma. per square inch, indicating that all of the manganese has been removed from the plate. A visual examination of the plate confirms that the manganese has been removed.

To determine the extent to which the plate is protected when treated in accordance with the present invention, it

is left in the acidic solution for about 19 hours. The stainless steel plate is removed, examined and the surface shows no evidence of corrosion or discoloration. Indeed, the surface of the plate appears as smooth and uniform as it did when new.

The stainless steel plate again is plated with an equivalent electrodeposit of manganese and returned to the same acidic solution. This time the potentiostat is not connected to the stainless steel plate. Rather, the plate is cleaned by the prior art method viz. allowed to remain in the acidic solution until the manganese has been digested by the acid. After about 20 minutes the plate is removed and found to have traces of manganese remaining thereon.

Further the plate is discolored, indicating that it is being i corroded by the acid.

Example II A type 201 stainless steel plate, suitable for use as a cathode in a cobalt electrodeposition process, is placed in an acidic solution simulating a cobalt electrodeposition anolyte and having the following composition:

Concentration, Substance: g./l. Co 15.0 Mn 3.0 Mg 20.0 Ni 0.2 Ca Saturated H 80 30.0

A polarization curve is obtained as shown in FIG. 3 and an anodic passive potential range is determined to be from about 200 mv. to 800 mv. with respect to a saturated calomel electrode.

The stainless steel plate is coated with cobalt and placed in the above described acidic solution. The plate then is treated in accordance with the present invention. A substantially constant potential of about 400 mv., with respect to an SCE, is maintained.

The initial current density is in excess of 100 ma. per square inch. Within about 7 minutes the current density abruptly decreases to less than .015 ma. per square inch indicating that all of the cobalt has been removed. The plate is removed and shows no signs of corrosion.

The foregoing example is repeated using a 316 stainless steel plate. The passive potential range is from about 150 mv. to 800 mv. with respect to an SCE. The plate is coated with cobalt and then treated in accordance with the present invention at a potential of about 200 mv.

The initial current density again exceeds 100 ma. per square inch. Within about 5 minutes the current density rapidly decreases to about 0.001 ma. per square inch indicating that all of the cobalt has been removed. The plate is removed and shows no signs of corrosion or discoloration.

Example III The procedure of Example II is repeated using a titanium plate and an acidic solution simulating an anolyte from a nickel electrodeposition process. The acidic solution has the following composition:

Concentration,

Substance: g./l. Ni 60.0

C1 60.0 H BO 15 0 H 50 300 An anodic passive potential range is determined to be from about 300 mv. to above about 2000 mv. with respect to a SCE as shown in FIG. 4.

The plate is coated with nickel, placed in the acidic solution and maintained at a passive potential of about 500 mv. Within about 10 minutes the current density required to maintain the plate at that potential rapidly decreases to about 0.025 ma. per square inch (all of the nickel has dissolved). The plate is left in the solution for a total time of about one hour. At the end of that time the plate is removed, examined, and found to show no signs of corrosion, discoloration or pitting.

Example IV The procedure of Example II is repeated using a titanium plate and an acidic solution simulating the anolyte from a copper electrodeposition process. The acidic solution has the following composition:

Substance: Concentration, g./l. Cu 10 H 62 Cl 0.1 Fe 4.5

An anodic passive potential range is determined as set forth hereinbefore and found to be from about 400 mv. to above about 2000 mv. with respect to a saturated calomel electrode as shown in FIG. .5.

The plate is coated with copper, placed in the acidic solution and maintained at a potential of about 500 mv. Within about 10 minutes the current density required to maintain the plate at that potential rapidly decreases to about 0.04 ma. per square inch (indicating all of the copper has dissolved). The plate is left in the solution for a total time of one hour. At the end of that time the plate is removed, examined and found to be free of discoloration or other evidence of corrosion.

Example V A zirconium alloy (Zircaloy-Z) plate is placed in an acidic solution comprising distilled water containing gm./liter sodium chloride and 10 gm./liter HCl. The temperature is maintained at 75 F. and a polarization curve is obtained as hereinbefore described.

From the polarization curve it is seen that there exists no anodic range of potentials at which the zirconium alloy is passive in this particular chloride environment. It is seen, however, that there is a range of cathodic potentials at which the zirconium alloy is passive, namely, from -500 mv. to 1000 mv. with respect to a SCE.

Manganese is electrodeposited on the zirconium plate and the plate then is placed in the acidic solution. A predetermined potential of -680 mv. SCE, within the cathodic passive potential range, is maintained on the plate. The initial current density to maintain that potential is about 350 milliamps per square inch. In less than 3 minutes the current density required to maintain the predetermined potential decreases to about 0.54 ma./in. and the plate is free of manganese. Further, the zirconium plate shows no indication of pitting or corrosion.

The foregoing example is repeated except without maintaining a controlled potential. It is found that, without a controlled potential, it takes from two to three times longer to dissolve an equivalent amount of manganese and the zirconium plate shows definite signs of pitting.

It is to be understood that the invention is not to be limited to the exact details of operation or the exact methods shown and described, as obvious modifications and equivalents will be apparent to those versed in the art.

What is claimed is:

1. A method of removing a metal contaminant plated on a different metal substrate, comprising:

(a) providing an acidic solution having a reference electrode and an applied voltage electrode therein,

(b) providing an adjustable voltage electrical power source,

(c) placing the metal substrate having the contaminant plated thereon into the acidic solution,

((1) connecting the power source to the metal substrate and the applied voltage electrode,

(e) providing a potential monitoring means adapted to measure a potential between the reference electrode and the metal substrate,

(f) continuously monitoring the potential between the reference electrode and the metal substrate, and

(g) applying a voltage to the metal substrate from the electrical power source, said voltage being suflicient to maintain a predetermined passive potential between the metal substrate and the reference electrode,

whereby the plated contaminant is removed from the metal substrate with substantially no detrimental effect on the surface of the metal substrate.

2. A method as set forth in claim 1 wherein the metal substrate is selected from the group consisting of stainless steel, zirconium, zirconium alloy, titanium and titanium alloy.

3. A method as set forth in claim 1 wherein the metal contaminant is selected from the group consisting of copper, nickel, chromium, cobalt and manganese.

4. A method as set forth in claim 1 wherein the acidic solution contains at least one acid selected from the group consisting of nitric, phosphoric and sulfuric.

5. A method as set forth in claim 1 wherein the metal substrate is selected from the group consisting of stainless steel, zirconium, titanium and titanium alloys, the metal contaminant is selected from the group consisting of copper, nickel, chromium, cobalt and manganese, and the acidic solution contains at least one acid selected from the group consisting of nitric, phosphoric and sulfuric.

6. A method as set forth in claim 5 wherein the metal substrate is stainless steel, the metal contaminant is manganese, the acidic solution is a solution of sulfuric acid, and the reference electrode is a standard reference selected from the group consisting of silver/silver chloride, mercury/mercury sulfate and saturated calomel electrodes.

7. A method as set forth in claim 6 wherein the predetermined potential is maintained within a range of from about 300 to 800 millivolts with respect to an SCE.

References Cited UNITED STATES PATENTS 3,525,702 8/1970 Von Sturm et a1. 204 2,596,307 5/1952 Stulfer 204-146 3,334,029 8/1967 Delafosse et a1 204 32 3,617,456 11/1971 Dillenberg 204146 FOREIGN PATENTS 656,697 1/1963 Canada 20414O OTHER REFERENCES Anodic Passivation Studies, vol. 16, No. 2, February 1960, issue of Corrosion, pp. 91-98.

THOMAS M. TUFARIELLO, Primary Examiner U.S. Cl. X.R. 204146 UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 3,826,724 Dated July 30, 1974 Inventor(s) Olen Lonnie Riggs, Jr. and Lyll Stanley Surtees It is certified that error appears in the above-ider1tified patent and that said Letters Patent are hereby corrected as shown below:

The assinee of the patent is Kerr-McGee Chemical Corporation Signed a nd sealed this 31st dayof' December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents =0RM PO-1050 (10-69) USCOMlM-DC 60376-P69 U.S, GOVERNMENT PRIN ING OFFICE: I959 0-366-334 

